Nipah (NiV) and Hendra (HeV) viruses are the deadliest human pathogens within the Paramyxoviridae family, which include human and animal pathogens of global biomedical importance. NiV and HeV are zoonotic viruses that can be transmitted directly to humans or intermediate amplifying hosts from their natural reservoirs in fruit bats. NiV and HeV infections cause respiratory and encephalitic illnesses, and NiV's mortality rate in humans can exceed 70 percent. Thus, NiV and HeV are the only paramyxoviruses classified BSL4 pathogens due to their extreme pathogenicity and lack of licensed vaccines or effective therapeutics. Paramyxoviruses are negative-sense RNA viruses that replicate in the cytoplasm. Their matrix structural protein organizes virion assembly at the plasma membrane and mediates viral budding from the cell surface. Curiously, the matrix protein of several paramyxoviruses has been observed to traffic in and out of the nucleus, although the functional relevance of paramyxovirus M proteins entering the nucleus has been unclear. We also found that NiV-M shuttles between the cytoplasmic and the nuclear compartments. However, our investigations established that proper nuclear-cytoplasmic trafficking of NiV-M, regulated by the ubiquitin-proteasome system, is critical for its functional localization to the plasma membrane, and its subsequent ability to mediate viral budding. Further study revealed a trove of NiV-M-interacting partners that places NiV-M at the nexus of multiple cell biological processes that are of contemporary scientific interest, including the cros regulation between the SUMO and ubiquitin-proteasome system in modulating subnuclear trafficking, and the function of PML bodies, nuclear structures involved in cellular antiviral defense. The goal of this application is to determine the role of SUMO and ubiquitin modifications in Nipah virus matrix trafficking and function. Understanding the basic cell biology of NiV-M will not only shed light on the properties that contribute to the unusually high virulence of this virus, but might also reveal host-pathogen interactions that have broader implications for viral pathogenesis in general. To understand the biological implications of NiV-M's complex intracellular sojourn, we propose the following three specific aims: (1) Identify and characterize post-translational modifications of NiV-M, focusing on the SUMO- and ubiquitin-proteasome pathway, and (2) define the functional roles for matrix post-translational modification in relation to viral pathogenesis.